1. Field of the Invention
The present invention relates to a joined body, a luminous vessel and an assembly for a high pressure discharge lamp.
2. Description of the Related Art
A high pressure discharge lamp with a discharge vessel of quartz has been widely used as a head light for an automobile due to its high brightness and light emission efficiency. The discharge vessel has a luminous portion and contains a light emitting gas inside of the vessel. The discharge vessel of such discharge lamp is made of quartz and thus transparent, so that the luminous portion may function as a point light source.
The assignee has filed a Japanese patent publication 2001-76677A, and disclosed a method of inserting pipes of molybdenum into openings at both ends, respectively, of a discharge vessel made of translucent alumina and of joining the pipes and the discharge vessel.
The discharge lamp described in Japanese patent publication 2001-76677A has been utilized as a pseudo point light source, for example, for an automobile. It has been, however, demanded a discharge lamp utilizing a transparent discharge vessel in fields including a head lamp for an automobile and a lamp for a projector.
Japanese patent publication 2001-519969A disclosed a method of sealing a luminous lamp made of sapphire and an end cap made of a polycrystalline alumina as a monolithic body.
Further, Japanese patent publication 56-44025B disclosed a composition of Dy2O3—Al2O3—SiO2 system for sealing a ceramic discharge vessel for a discharge system.
Further, U.S. Pat. No. 3,588,573 disclosed a method of sealing high temperature vapor using an eutectic cement of Dy2O3—Al2O3 system.
According to the method described in Japanese patent publication 2001-519969A, the luminous vessel made of sapphire is joined with the end cap made of polycrystalline alumina by solid phase reaction. The inventors, however, have studied the joined body and encountered the following problems. That is, according to the method, grains of polycrystalline alumina may be easily grown along an interface between sapphire and polycrystalline alumina. When the growth of the grains of polycrystalline alumina is terminated before the grains are grown to a single crystal (sapphire), the resulting alumina grains are left along the joining interface. The mechanical strength at the joining interface is thus reduced, so that the joined body may be broken at the joining interface by a thermal stress, which does not otherwise result in the fracture of the joined body. The control of growth of the polycrystalline alumina grains is difficult along the interface of sapphire and polycrystalline alumina, because the growth of the grains is susceptible to sintering conditions or the like.